COLUMBIA — "Happy." That's how Tim Wallace, 23, says his work at Central Missouri Subcontracting Enterprises makes him feel.  He gets a ride from his supervisor to the sheltered workshop at 4040 S. Bearfield Road every weekday, arriving around 8:30 a.m. For the next six hours, he and two colleagues often sit at a table with a trough of hundreds of Nasopure salt packets in front of them. On a recent Monday, Christian rock played on the intercom as they went to work, counting out 20 of the packets at a time and placing them into zippered plastic baggies.